Having broken my fair share of bolts over the years, may I chime in?
First, you may want to consider an impact gun (a small, electric one from Sears works wonders, delivering claimed 700 inch pounds - yeah, it doesn't but they can dream it does). I use it all the time now on stubborn bolts and screws, especially old, seized Phillips head stuff.
What you have is old steel bolts rusted (likely) in the aluminum of the caliper. Not the best combo, but a good one for a disaster. I would apply PB Blaster, let soak in, then try to turn the bolts by TIGHTENING them a fraction of a turn. If it moves, then back it out. If not, then try the impact driver. The impacts and twisting works wonders on breaking the bond between two metals. If you don;t have one, use PB and heat or hit it with a hammer a few times (not hard enough to deform the bolt heads). It will break the bond.
Good luck. I tend to work on bikes that sat for 20+ years in yards, so this is what I deal with more often than I'd like to.
If you end up snapping a bolt, forget about screw extractors - you break one of those and you have a major headache on your hands. Instead, get some left turn drill bits and try to drill it out. More often then not, the heat from the drilling loosens the bolts and removes them. But, make sure you have a nice centered hole in case you have to drill it out completely without damaging threads.
Patience and slow is the key here. Good luck.